Add explicit exception for handlers to delay messages

* Adds exception RetryableMessageHandlerException, that a message handler can throw, to indicate that a message cannot be handled right now.
* Examples of this could be handling a message that, in turn, requires other messages to have been processed first. In those situations, the handler can then throw RetryableMessageHandlerException, which will make the QueueProcessor retry the message later, when the message visibility has passed.
Merge pull request #25 from iZettle/retry-messages-later

Add explicit exception for handlers to delay messages